1. Field of the Invention
Embodiments of the invention relate generally to the field of communications networking. More particularly, an embodiment of the invention relates to methods of and apparatus for “Smart” RF over Glass (RFoG) CPE Unit with Seamless PON Upgrade Capability.
2. Discussion of the Related Art
Telephone companies such as Verizon and AT&T have started to offer services over fiber-to-the-premise (FTTP) and fiber-to-the-curb (FTTC) systems such as FiOS™ and U-Verse™. These systems offer dramatically higher data bandwidths by bringing optical fiber to the home or close to home. In order to maintain their upper hand in bandwidth per customer, North American cable operators started deploying scalable fiber-to-the-home (FTTH) systems, building upon fiber deployed to date in new builds and upgrades that can offer similar to, or higher than, bandwidths provided by FiOS™ and U-Verse™.
MSOs want to continue utilizing DOCSIS platform for wideband services such as high speed data, Voice over IP (VoIP) and other services supported by this platform, which provides for downstream data bandwidth up to 640 Mb/s or more, until such a time as yet higher data speeds are required. At such a time, the MSOs want the flexibility to upgrade their FTTH CPE device to handle Gb/s data speeds offered by passive optical networks (PONs) such as GPON or GEPON. They also want to support deployed interactive TV services that are based on set top boxes with active upstream signaling to support fully interactive services such as Video on Demand (VoD) and Switched Digital Video (SDV).
RF over Glass (RFoG) is the name given to the generic FTTH architecture that supports both legacy DOCSIS cable upstream signals and an optional future expansion to additional high speed (>1 Gb/s) PON service. However, deploying cost-effective RFoG system makes future expansion of this system with GPON or GEPON more difficult. The RFoG transmitters used to transmit upstream DOCSIS signals and set top box upstream signaling information for interactive TV, and placed in the CPE utilize a low-cost 1310 nm laser, which is the same wavelength as that used by upstream PON signals. The solution has been to use a different wavelength, usually 1590 nm, to transport the cable upstream signal and 1310 nm to transport the upstream PON signal. For systems that initially deployed 1310 nm upstream lasers, the expansion would result in replacing and obsolescing these deployed lasers with much higher cost CPE devices.
FIG. 1 shows the schematic diagram of one type (“Version A”) of customer-premise-equipment (CPE) device used by cable operators building an RFoG system. The Version A CPE device is a low-cost device that provides only traditional cable services and no PON. The CPE device uses an optical filter to separate the downstream 1550 nm signal from the upstream 1310 nm signal. This filter is not deployed if two fibers are used, one for downstream and one for upstream. The CPE device uses a relatively low-cost 1310 nm laser for transmitting upstream signals and an optical receiver for detecting the downstream 1550 nm signal. The two paths are combined using a RF diplex filter onto the home coaxial cable.
FIG. 2 shows the schematic diagram of a higher-cost RFoG CPE device (“Version B”). This CPE also supports only traditional cable services and no PON, but uses a 1590 nm laser to transport the upstream signals. The advantage of a Version B CPE device is that it uses 1590 nm for the cable upstream and hence does not conflict with the 1490 nm/1310 nm (down/up) wavelengths standardized in a PON system. Consequently, Version B can offer a PON upgrade port for the future addition of a PON system.
A disadvantage of the Version B CPE device is that 1590 nm lasers are currently significantly higher in price than 1310 nm lasers due to lower demand and higher performance requirements. Cable operators have to decide whether to start with the Version A CPE, which requires lower initial capital expenditures (capex) but has to be replaced when PON service has to be added, or with Version B, which requires a higher initial capex but does not need to be replaced when PON service has to be added at some point in the future. Since initial PON penetration rates are expected to be low, and due to the importance placed on low initial capital expenditure, many cable operators will start with the Version A CPE even though this will result in stranded capex at some point in the future when they have to implement PON service.
FIG. 3 shows a third version of the RFoG CPE device (“Version C”) which represents the Version B CPE device upgraded to PON service by the addition of an Optical Network Unit (ONU). The PON ONU transmits upstream at 1310 nm and receives downstream signals at 1490 nm. A second optical filter is used to separate the two PON wavelengths.
The Version C CPE device would be used to provide both traditional cable services and PON service to residences. It could also be used to provide PON service to small-to-medium businesses (SMBs) although in this case the traditional cable portion of the CPE device (carried by the 1310 nm laser) would likely not be used and would represent wasted capex.
The quandary faced by cable operators building FTTH systems is that regardless of what version of RFoG CPE devices currently available they choose to use (Version A, B or C) the result will be either stranded capex (when upgrading to PON from Version A) or higher initial capex (when starting from Version B) or wasted capex (when using Version C for business services).